In a mass spectrometer, a series of ion guide devices is necessary to realize effective ion transmission from an ion source which is typically at relatively high gas pressure (1-105Pa) to the mass analyzer which is at low pressure (<1 Pa). The ion guide device generally consists of electrodes on which radio-frequency voltages are applied. The radio-frequency voltages form a pseudo-potential barrier inside the device to confine or focus ions in radial direction with exists of collisions with the buffer gas. Meanwhile, with the gas flow induced by the vacuum interface or a DC electric field applied along the axis of the device, the ions can be transferred along the axial direction and finally get into the next stage at higher vacuum, and then are analyzed by a mass analyzer. Some radio-frequency ion guiding devices, e.g. the multipole ion guide (U.S. Pat. No. 5,179,278) by D. J. Douglas and the surface reflection ion guide (U.S. Pat. No. 5,572,035) by J. Franzen, can guide the ions at relatively low pressure; And later, the Q-array guide invented by N. Inatsugu and H. Waki, and the ion tunnel by Bateman et al., can guide the ions under the pressure of less than 5 torr. To guide as well as to focus the ions at higher pressure, R. D. Smith proposed the ion funnel device (U.S. Pat. No. 6,107,628), which may effectively transfer and focus the ions at the pressure of approximately 30 torr. The sensitivity of the instrument can be improved by coupling this ion funnel to the atmosphere pressure interface.
However, partly due to the funnel shaped structure, the strong gas flow induced from the atmosphere pressure interface, which is usually a capillary or orifice, exists on the whole length of ion funnel along the axis. Even if by adding a jet-disrupter before the ion funnel, the strong gas flow cannot be totally avoided, especially near the exit of the ion funnel. The gas flow into the next vacuum stage requires more powerful and more expensive pumping system. Also, those neutral gas molecules produce more noise on the detector. Particularly, when the ion funnel is matched with an electrospray ion source, those droplets which are not sufficiently desolvated and which are carried over as the gas flow will bring more neutral noise. Sensitivity of the instrument will be reduced. In other words, the travelling direction of the ions in the ion funnel is coaxial with the direction of neutral components (charged droplets may be approximately regarded as neutral due to too large mass-to-charge ratio), so noise is introduced, and a more expensive vacuum pump with higher pumping capability is also needed. Another issue in ion funnel is the low mass discrimination due to the axial trapping effect at the exit. The smaller diameter of ring electrode at the exit, the stronger axial trapping field will be produced to avoid the effective transmission to ions. Thus, the diameter of the last ring of ion funnel is generally not less than 1.5 mm, which thus brings a high burden to the vacuum system in the next stage.
An off-axis transmission device was designed by K. Giles in US2011/0049357. The device is formed by coupling a tunnel consisted of stacked-ring electrodes which has a larger diameter and another tunnel which has a smaller diameter. A DC bias is applied between those two conjoined tunnels. Ions enter from the tunnel with the large diameter and then are guided into the tunnel with smaller diameter in an off-axis way. Neutral gas molecules are pumped away along the axis of the large tunnel. Signal to noise ratio can be improved remarkably by this ion guide device. There are two possible issues for this design. The first one is the ion focusing effect is limited by the diameter of the smaller tunnel. If one wants to get good focusing and uses very small diameter, the RF barrier at the conjoined zone becomes strong and it does not facilitate to the ion transmission. The second one is structure of device is relatively complicated and it will be not easy to manufacture.